The invention relates to an explosive plug for blocking tubes. The plug is suitable in general for tubes of boilers, steam generators and heat exchangers. The plug is more particularly suitable for blocking the tubes of steam generators forming a part of a nuclear power station.
When a steam generator tube, for example, has a leak or a threatened leak, it has to be put out of operation by stopping up each end. The ends of a faulty tube are stopped by means of explosive plugs. An explosive plug substantially comprises a tubular metal body closed at one end by a solid head and open at the other end. The metal body has a surface-contact region extending from its open end and containing an explosive charge which can be fired by a detonator. During operation, the metal body is inserted and engages in the end of the faulty tube. Next, the charge is exploded and the surface-contact region of the metal tube is spattered against the end of the deflective tube at a pressure and temperature such that the metal or alloys forming the surface-contact region and the aforementioned end are chemically bonded and form a metallurgical weld.
A first known plug for blocking the ends of tubes in a steam generator or the like has a metal body and a surface-contact region which is frusto-conical and flared towards the solid head. The surface-contact region contains a single explosive charge. The detonator is disposed in the metal body at the end opposite the head relative to the explosive charge. In order to block the end of a defective tube, using the known plug, the metal body is mounted in the end of the tube, which has previously been frusto-conically widened towards the exterior. During the assembly process, the frusto-conical surface-contact region of the metal body is placed opposite the flared part of the aforementioned end. When the charge explodes, the surface-contact region is bent back and welded to the flared part of the end in question.
The first known plug has disadvantages. It is difficult to position the plug in the end of the tube before the explosive and hold it there. The surface-contact region is applied to the flared end of the tube by spattering the metal or alloy forming the aforementioned region at a given angle corresponding to the conicity of the aforementioned end. In other words, the metal or alloy in the surface-contact region is spattered against the flared end of the tube from distances which increase in proportion to the common longitudinal axis of the surface-contact region and the tube. Consequently, the metal or alloy in the surface-contact region may be spattered along the flared end of the tube, since the variation in pressures and temperatures is such that the metal weld between the metal body and the end may be defective. The weld is even less reliable in that it is relatively short and extends only along the limited length of the flared end of the tube.
A second known plug has a metal body and a surface-contact region which is likewise frusto-conical and contains a single explosive charge. As before, the detonator is mounted at the end remote from the head of the metal body relative to the explosive charge. When the plug is used, the end of the faulty tube which is to be blocked is not widened and remains cylindrical. When the metal body is mounted in the aforementioned end, the surface-contact region is placed level therewith, its wider part facing the interior of the tube. When the charge explodes, the surface-contact region is bent back and welded to the aforementioned end as a result of spattering of metal or alloy at an angle defined by the conicity of the surface-contact region. Consequently, the second known plug has substantially the same disadvantages as the first.